It's a figure of speech meant to "test the absolute limits." Having absolute conviction and determination goes further than actual ability, sometimes.
We really don't know what the limits of our abilities are unless we test them. And, what if we all believed it when someone said that we could not do something, for some very plausible sounding reason?
The idea is that, there are things that, on the surface appear to be out of reach, but how do you REALLY know unless you believe in your heart you can reach them and you approach it that way.
Example: One of the least intellectually gifted (not very smart) friends of mine is a psychiatrist. I no genius, but feel that I am at least 2-3x smarter than she is, naturally. However, she believed that she could become a psychiatrist, was motivated to study night and day, and she did. I would have bet that she did not have the ability, no way.
If SHE can become an M.D. with the psychiatry sub specialty, I am not sure what is really impossible - if the motivation is there.